1. Field of the Invention
The present invention relates to an efficient gripping of opposite end surfaces of a log or timber in a veneer lathe, and more particularly to a method and apparatus for rotary-cutting a timber in a veneer lathe in which when the timber is cut, end faces of the timber close to the outer peripheral surface thereof to be rotary-cut are gripped, and the part to be gripped is changed stepwise as the diameter of the timber reduces so as to enable effective cutting.
2. Related Art
The veneer lathe is a machine wherein a log or timber is rotated about its longitudinal axis while a knife is applied to the outer peripheral surface thereof to rotary-cut the timber so that a veneer of uniform thickness is continuously peeled off from the outer peripheral surface. As will be described in more detail, the veneer lathe has two types, namely, a center drive type which grips and rotates opposite end faces of a timber, and an outer periphery drive type in which a rotational drive roll having thrust-protrusions is arranged above a knife for cutting the timber to transmit drive of the drive roll to the outer peripheral surface of the timber.
The veneer lathe of the center drive type is preferred to the veneer lathe of the outer periphery drive type because the peripheral surface of the timber is not damaged due to its rotational drive, as will be described later. In the veneer lathe of the center drive type, pawls of chucks bite into opposite end faces of the timber to rotatably grip the timber. During the cutting of the timber, a cutting resistance force acts in a peripheral direction of the timber as the outer peripheral surface thereof is rotary-cut by a knife. It is thus necessary to rotate and drive the timber during the cutting against a cutting moment which is the product of the aforesaid cutting resistance force and the radius of the timber.
However, since the diameter of the timber is large particularly at the outset of the cutting, the cutting moment is also large. The pawls of the chucks need to impart to the timber a resisting moment enough to overcome such a large cutting moment. However, since the radius of the pawls of the chucks, as measured from the center axis of the timber, is relatively small, it is required that an extremely large resistance force be generated in the pawls of the chucks in order to generate a large resisting moment. However how strong the pawls may be, the end faces of the timber into which the pawls bite sometimes cannot withstand a large resistance force, thus resulting in a breakage.